Cellular Respiration · 细胞呼吸

Cellular respiration is the metabolic pathway through which living cells release energy from organic molecules and convert it into ATP, the universal energy currency of life. In A-Level Biology, respiration spans four interconnected stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Understanding how each stage contributes to the net yield of ATP is essential for exam success.

细胞呼吸是活细胞从有机分子中释放能量并将其转化为ATP（生命的通用能量货币）的代谢途径。在A-Level生物学中，呼吸作用涵盖四个相互关联的阶段：糖酵解、连接反应、克雷布斯循环和氧化磷酸化。理解每个阶段对ATP净产量的贡献对于考试成功至关重要。

1. Overview: Aerobic vs Anaerobic Respiration · 有氧与无氧呼吸概述

Respiration can be aerobic (requiring oxygen) or anaerobic (proceeding without oxygen). Aerobic respiration involves all four stages and produces up to 38 ATP molecules per glucose molecule, though the actual yield is closer to 30-32 ATP due to membrane leakage and the energetic cost of transporting NADH into the mitochondria. Anaerobic respiration involves only glycolysis followed by a fermentation pathway that regenerates NAD⁺, yielding just 2 ATP per glucose molecule.

呼吸可以分为有氧呼吸（需要氧气）和无氧呼吸（在无氧条件下进行）。有氧呼吸涉及全部四个阶段，每个葡萄糖分子最多产生38个ATP分子，但由于膜泄漏和将NADH运入线粒体的能量成本，实际产量接近30-32个ATP。无氧呼吸仅涉及糖酵解及随后的发酵途径来再生NAD⁺，每个葡萄糖分子仅产生2个ATP。

2. Glycolysis: The Universal First Stage · 糖酵解：通用的第一阶段

Glycolysis occurs in the cytoplasm of all living cells and does not require oxygen. A six-carbon glucose molecule (C₆H₁₂O₆) is phosphorylated twice using 2 ATP, producing hexose bisphosphate, which then splits into two three-carbon triose phosphate (TP) molecules. Each TP is oxidised to pyruvate (C₃H₄O₃) through a series of enzyme-catalysed reactions, with dehydrogenase enzymes removing hydrogen atoms that are accepted by NAD⁺ to form reduced NAD (NADH).

糖酵解发生在所有活细胞的细胞质中，不需要氧气。一个六碳葡萄糖分子（C₆H₁₂O₆）使用2个ATP进行两次磷酸化，生成己糖二磷酸，然后分裂为两个三碳的磷酸丙糖（TP）。每个TP通过一系列酶催化反应被氧化为丙酮酸（C₃H₄O₃），脱氢酶去除氢原子，由NAD⁺接受形成还原型NAD（NADH）。

The net yield from glycolysis is 2 ATP (4 produced minus 2 invested), 2 reduced NAD, and 2 pyruvate molecules per glucose. The ATP is produced by substrate-level phosphorylation, where a phosphate group is directly transferred from a substrate molecule to ADP. This stage is common to both aerobic and anaerobic respiration and is therefore considered the most ancient metabolic pathway.

糖酵解的净产量为每个葡萄糖产生2个ATP（产生4个减去投入2个）、2个还原型NAD和2个丙酮酸分子。ATP通过底物水平磷酸化产生，即磷酸基团直接从底物分子转移到ADP。这一阶段在有氧和无氧呼吸中都是相同的，因此被认为是最古老的代谢途径。

3. The Link Reaction: Bridging Cytoplasm and Mitochondrion · 连接反应：连接细胞质与线粒体

In aerobic respiration, each pyruvate molecule enters the mitochondrial matrix via active transport. The link reaction, catalysed by the pyruvate dehydrogenase complex, converts each pyruvate into acetyl coenzyme A (acetyl-CoA). During this process, pyruvate is decarboxylated (CO₂ is removed) and dehydrogenated (hydrogen atoms are removed and accepted by NAD⁺ to form NADH). The remaining two-carbon acetyl group combines with coenzyme A to form acetyl-CoA.

在有氧呼吸中，每个丙酮酸分子通过主动运输进入线粒体基质。连接反应由丙酮酸脱氢酶复合体催化，将每个丙酮酸转化为乙酰辅酶A（乙酰-CoA）。在此过程中，丙酮酸发生脱羧（去除CO₂）和脱氢（去除氢原子，由NAD⁺接受形成NADH）。剩余的二碳乙酰基与辅酶A结合形成乙酰-CoA。

Since one glucose molecule yields two pyruvate molecules, the link reaction occurs twice per glucose, producing 2 acetyl-CoA, 2 CO₂, and 2 reduced NAD. No ATP is produced directly in this stage. The acetyl-CoA then enters the Krebs cycle by combining with oxaloacetate, a four-carbon compound, to form citrate.

由于一个葡萄糖分子产生两个丙酮酸分子，每个葡萄糖的连接反应发生两次，产生2个乙酰-CoA、2个CO₂和2个还原型NAD。此阶段不直接产生ATP。乙酰-CoA随后通过草酰乙酸（一种四碳化合物）结合形成柠檬酸进入克雷布斯循环。

4. The Krebs Cycle: The Metabolic Hub · 克雷布斯循环：代谢枢纽

The Krebs cycle (also called the citric acid cycle or TCA cycle) takes place in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through a series of decarboxylation and dehydrogenation reactions, citrate is progressively oxidised back to oxaloacetate, releasing two CO₂ molecules per turn. Each turn also produces 3 reduced NAD, 1 reduced FAD, and 1 ATP by substrate-level phosphorylation.

克雷布斯循环（也称为柠檬酸循环或TCA循环）发生在线粒体基质中。乙酰-CoA（2C）与草酰乙酸（4C）结合形成柠檬酸（6C）。通过一系列的脱羧和脱氢反应，柠檬酸逐步氧化回到草酰乙酸，每轮释放两个CO₂分子。每轮还产生3个还原型NAD、1个还原型FAD和1个ATP（通过底物水平磷酸化）。

The Krebs cycle turns twice per glucose molecule (once for each acetyl-CoA), yielding a total of 6 reduced NAD, 2 reduced FAD, 2 ATP, and 4 CO₂. Combined with the link reaction, the total CO₂ released before the electron transport chain is 6 molecules per glucose. The reduced coenzymes (NADH and FADH₂) carry high-energy electrons to the inner mitochondrial membrane for the final stage.

克雷布斯循环每个葡萄糖分子转两轮（每个乙酰-CoA一轮），总共产生6个还原型NAD、2个还原型FAD、2个ATP和4个CO₂。加上连接反应，在电子传递链之前每个葡萄糖释放的CO₂总量为6个分子。还原型辅酶（NADH和FADH₂）将高能电子携带到线粒体内膜以进行最后阶段。

5. Oxidative Phosphorylation: The ATP Powerhouse · 氧化磷酸化：ATP发电站

Oxidative phosphorylation occurs across the inner mitochondrial membrane and consists of two linked processes: the electron transport chain (ETC) and chemiosmosis. Reduced NAD and reduced FAD donate electrons to the ETC, a series of protein complexes (I, II, III, IV) embedded in the inner membrane. As electrons pass along the chain, each carrier is at a progressively lower energy level, and the energy released is used to pump protons (H⁺) from the matrix into the intermembrane space, establishing an electrochemical gradient.

氧化磷酸化发生在线粒体内膜上，由两个相连的过程组成：电子传递链（ETC）和化学渗透。还原型NAD和还原型FAD将电子捐赠给ETC，ETC是嵌入内膜的一系列蛋白质复合体（I、II、III、IV）。随着电子沿链传递，每个载体处于逐渐降低的能级，释放的能量用于将质子（H⁺）从基质泵入膜间隙，建立电化学梯度。

The protons flow back into the matrix through ATP synthase, a channel protein that couples proton flow to ATP synthesis ： this is chemiosmosis. Each reduced NAD yields approximately 2.5 ATP and each reduced FAD yields approximately 1.5 ATP. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. Without oxygen, the ETC halts, protons are no longer pumped, and ATP synthesis stops ： explaining why aerobic respiration depends on oxygen.

质子通过ATP合酶（一种将质子流动与ATP合成耦合的通道蛋白）流回基质：这就是化学渗透。每个还原型NAD产生约2.5个ATP，每个还原型FAD产生约1.5个ATP。氧气作为最终的电子受体，与电子和质子结合形成水。没有氧气，ETC停止，质子不再被泵出，ATP合成停止：这解释了为什么有氧呼吸依赖于氧气。

6. Anaerobic Respiration: Surviving Without Oxygen · 无氧呼吸：在无氧条件下生存

When oxygen is unavailable, cells cannot run the ETC. NADH accumulates because it cannot be reoxidised by the electron transport chain, and glycolysis halts when the cell runs out of NAD⁺. To overcome this, cells use fermentation pathways that oxidise NADH back to NAD⁺, allowing glycolysis to continue producing 2 ATP per glucose. In animal cells and some bacteria, pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD⁺ in the process.

当氧气不可用时，细胞无法运行ETC。NADH积累因为它不能被电子传递链重新氧化，当细胞耗尽NAD⁺时糖酵解停止。为了克服这一问题，细胞使用发酵途径将NADH氧化回NAD⁺，使糖酵解能够继续每个葡萄糖产生2个ATP。在动物细胞和一些细菌中，丙酮酸被乳酸脱氢酶还原为乳酸，在此过程中再生NAD⁺。

In plants, yeast, and some microorganisms, pyruvate is first decarboxylated to ethanal (acetaldehyde), releasing CO₂, and then reduced to ethanol by alcohol dehydrogenase, regenerating NAD⁺. This is alcoholic fermentation, exploited industrially in brewing and baking. Both pathways produce only 2 ATP per glucose ： the remaining energy remains locked in the partially oxidised products (lactate or ethanol), making anaerobic respiration far less efficient than aerobic respiration.

在植物、酵母和一些微生物中，丙酮酸首先脱羧生成乙醛，释放CO₂，然后被乙醇脱氢酶还原为乙醇，再生NAD⁺。这就是酒精发酵，在酿造和烘焙工业中得到应用。两种途径每个葡萄糖仅产生2个ATP：剩余能量仍锁定在部分氧化的产物（乳酸或乙醇）中，使无氧呼吸的效率远低于有氧呼吸。

7. Respiratory Substrates and Respiratory Quotient (RQ) · 呼吸底物与呼吸商

While glucose is the primary respiratory substrate, cells can also respire lipids, proteins, and other carbohydrates. The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed during respiration. For pure carbohydrate respiration, RQ = 1.0 (equal volumes of CO₂ produced and O₂ consumed). For lipids, which are more reduced than carbohydrates, more O₂ is required per CO₂ produced, giving an RQ of approximately 0.7. For proteins, the RQ is around 0.9, reflecting their intermediate oxidation state.

虽然葡萄糖是主要的呼吸底物，细胞也可以呼吸脂质、蛋白质和其他碳水化合物。呼吸商（RQ）是呼吸过程中产生的CO₂与消耗的O₂之比。对于纯碳水化合物呼吸，RQ = 1.0（产生的CO₂与消耗的O₂体积相等）。对于脂质（比碳水化合物还原程度更高），每产生一个CO₂需要更多的O₂，RQ约为0.7。对于蛋白质，RQ约为0.9，反映其中间的氧化状态。

RQ values provide experimental insight into which substrate an organism is predominantly respiring. A germinating seed with an RQ of 1.0 is respiring carbohydrates (starch), while an RQ below 0.8 suggests lipid mobilisation, common in oil-rich seeds like sunflower and castor bean. RQ can exceed 1.0 during anaerobic respiration when CO₂ is produced without corresponding O₂ consumption, or when organic acids are the substrate.

RQ值通过实验揭示了生物体主要呼吸的底物类型。RQ为1.0的萌发种子正在呼吸碳水化合物（淀粉），而RQ低于0.8表明脂质动员，这在向日葵和蓖麻等富含油脂的种子中常见。在无氧呼吸期间（产生CO₂但没有相应的O₂消耗），或以有机酸为底物时，RQ可以超过1.0。

8. Factors Affecting Respiration Rate · 影响呼吸速率的因素

Respiration rate is influenced by temperature, oxygen concentration, substrate availability, and the metabolic demands of the cell. Temperature affects enzyme activity: as temperature increases, kinetic energy rises and respiration rate increases until enzymes begin to denature above their optimum (typically 35-40°C for most mammalian enzymes). Below the optimum, respiration rate approximately doubles for every 10°C rise (Q₁₀ ≈ 2), consistent with the Arrhenius equation for enzyme-catalysed reactions.

呼吸速率受温度、氧气浓度、底物可用性和细胞代谢需求的影响。温度影响酶活性：随着温度升高，动能增加，呼吸速率增加，直到酶在其最适温度以上开始变性（大多数哺乳动物酶的最适温度为35-40°C）。在最适温度以下，呼吸速率大约每升高10°C翻一番（Q₁₀ ≈ 2），这与酶催化反应的阿伦尼乌斯方程一致。

Oxygen concentration is a limiting factor for aerobic respiration ： below a critical threshold, the ETC slows because oxygen is the final electron acceptor. This explains why waterlogged soils cause root death: oxygen diffusion through water is 10,000 times slower than through air. In respirometer experiments, potassium hydroxide (KOH) is used to absorb CO₂ so that the volume change in the capillary tube reflects only O₂ consumption, allowing precise measurement of respiration rate.

氧气浓度是有氧呼吸的限制因素：低于临界阈值时，由于氧气是最终的电子受体，ETC减慢。这解释了为什么涝渍土壤导致根系死亡：氧气在水中的扩散速度比在空气中慢10,000倍。在呼吸计实验中，使用氢氧化钾（KOH）吸收CO₂，使毛细管中的体积变化仅反映O₂消耗，从而精确测量呼吸速率。

9. ATP Yield Summary: Tracking Every Molecule · ATP产量总结：追踪每个分子

A complete accounting of ATP production from one glucose molecule reveals why aerobic respiration is so efficient. Glycolysis yields 2 ATP (substrate-level) and 2 NADH, which produce 5 ATP via the ETC. The link reaction yields 2 NADH producing 5 ATP. The Krebs cycle yields 2 ATP (substrate-level), 6 NADH producing 15 ATP, and 2 FADH₂ producing 3 ATP. The theoretical total is 32 ATP per glucose, though the actual yield in eukaryotic cells is 30 ATP because the 2 NADH from glycolysis must be shuttled into the mitochondria at a cost of approximately 1 ATP each.

对一个葡萄糖分子的ATP产量进行完整核算，揭示了为什么有氧呼吸如此高效。糖酵解产生2个ATP（底物水平）和2个NADH，通过ETC产生5个ATP。连接反应产生2个NADH，产生5个ATP。克雷布斯循环产生2个ATP（底物水平）、6个NADH产生15个ATP、2个FADH₂产生3个ATP。理论总产量为每个葡萄糖32个ATP，但在真核细胞中，实际产量为30个ATP，因为来自糖酵解的2个NADH必须被转运到线粒体中，每个大约消耗1个ATP。

10. Key Bilingual Terms · 关键双语术语

11. Exam Tips for A-Level Respiration Questions · A-Level呼吸作用考题技巧

When answering respiration questions, always specify the location of each stage (cytoplasm, mitochondrial matrix, or inner mitochondrial membrane) ： examiners award marks for correct compartmentalisation. Use precise terminology: “substrate-level phosphorylation” not “ATP made directly”, and “chemiosmosis” not “protons make ATP”. For the ETC, describe the role of oxygen explicitly as the final electron acceptor that combines with electrons and protons to form water.

在回答呼吸作用问题时，始终指明每个阶段的位置（细胞质、线粒体基质或线粒体内膜）：考官对正确的区室划分给分。使用精确的术语：”底物水平磷酸化”而不是”直接产生ATP”，”化学渗透”而不是”质子制造ATP”。对于ETC，明确描述氧气作为最终电子受体的作用，它与电子和质子结合形成水。

Common exam pitfalls include confusing NAD with NADP (NADP is used in photosynthesis, NAD in respiration), forgetting that glycolysis occurs in the cytoplasm (not the mitochondrion), and stating that oxygen is “used to make CO₂” when CO₂ is actually produced during decarboxylation in the link reaction and Krebs cycle. For anaerobic respiration, emphasise that the purpose of fermentation is to regenerate NAD⁺, not to produce ATP ： the ATP comes from glycolysis alone.

常见的考试陷阱包括混淆NAD与NADP（NADP用于光合作用，NAD用于呼吸作用），忘记糖酵解发生在细胞质中（而不是线粒体），以及错误地声称氧气”用于制造CO₂”，而实际上CO₂是在连接反应和克雷布斯循环中的脱羧过程中产生的。对于无氧呼吸，强调发酵的目的是再生NAD⁺，而不是产生ATP：ATP仅来自糖酵解。

Cellular Respiration · 细胞呼吸